* cache write back & barriers
@ 2013-06-12 8:03 folkert
2013-06-13 8:26 ` Stefan Hajnoczi
2013-06-13 14:40 ` Alexandre DERUMIER
0 siblings, 2 replies; 6+ messages in thread
From: folkert @ 2013-06-12 8:03 UTC (permalink / raw)
To: kvm
Hi,
In virt-manager I saw that there's the option for cache writeback for
storage devices.
I'm wondering: does this also make kvm to ignore write barriers invoked
by the virtual machine?
regards,
Folkert van Heusden
--
Always wondered what the latency of your webserver is? Or how much more
latency you get when you go through a proxy server/tor? The numbers
tell the tale and with HTTPing you know them!
http://www.vanheusden.com/httping/
-----------------------------------------------------------------------
Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: cache write back & barriers 2013-06-12 8:03 cache write back & barriers folkert @ 2013-06-13 8:26 ` Stefan Hajnoczi 2013-06-13 8:47 ` folkert 2013-06-13 14:40 ` Alexandre DERUMIER 1 sibling, 1 reply; 6+ messages in thread From: Stefan Hajnoczi @ 2013-06-13 8:26 UTC (permalink / raw) To: folkert; +Cc: kvm On Wed, Jun 12, 2013 at 10:03:10AM +0200, folkert wrote: > In virt-manager I saw that there's the option for cache writeback for > storage devices. > I'm wondering: does this also make kvm to ignore write barriers invoked > by the virtual machine? No, that would be unsafe. When the guest issues a flush then QEMU will ensure that data reaches the disk with -drive cache=writeback. Stefan ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: cache write back & barriers 2013-06-13 8:26 ` Stefan Hajnoczi @ 2013-06-13 8:47 ` folkert 2013-06-14 10:53 ` Stefan Hajnoczi 0 siblings, 1 reply; 6+ messages in thread From: folkert @ 2013-06-13 8:47 UTC (permalink / raw) To: Stefan Hajnoczi; +Cc: kvm Hi, > > In virt-manager I saw that there's the option for cache writeback for > > storage devices. > > I'm wondering: does this also make kvm to ignore write barriers invoked > > by the virtual machine? > > No, that would be unsafe. When the guest issues a flush then QEMU will > ensure that data reaches the disk with -drive cache=writeback. Aha so the writeback behaves like the consume harddisks with write-cache on them. In that case maybe an extra note could be added to the virt-manager (excellent software by the way!) that if the client vm supports barriers, that write-back in that case then is safe. Agree? Folkert van Heusden -- Ever wonder what is out there? Any alien races? Then please support the seti@home project: setiathome.ssl.berkeley.edu ---------------------------------------------------------------------- Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: cache write back & barriers 2013-06-13 8:47 ` folkert @ 2013-06-14 10:53 ` Stefan Hajnoczi 2013-06-16 12:06 ` [virt-tools-list] " Richard W.M. Jones 0 siblings, 1 reply; 6+ messages in thread From: Stefan Hajnoczi @ 2013-06-14 10:53 UTC (permalink / raw) To: folkert; +Cc: kvm, virt-tools-list On Thu, Jun 13, 2013 at 10:47:32AM +0200, folkert wrote: > Hi, > > > > In virt-manager I saw that there's the option for cache writeback for > > > storage devices. > > > I'm wondering: does this also make kvm to ignore write barriers invoked > > > by the virtual machine? > > > > No, that would be unsafe. When the guest issues a flush then QEMU will > > ensure that data reaches the disk with -drive cache=writeback. > > Aha so the writeback behaves like the consume harddisks with write-cache > on them. > In that case maybe an extra note could be added to the virt-manager > (excellent software by the way!) that if the client vm supports > barriers, that write-back in that case then is safe. Agree? CCed virt-manager mailing list so they can see your request. Stefan ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [virt-tools-list] cache write back & barriers 2013-06-14 10:53 ` Stefan Hajnoczi @ 2013-06-16 12:06 ` Richard W.M. Jones 0 siblings, 0 replies; 6+ messages in thread From: Richard W.M. Jones @ 2013-06-16 12:06 UTC (permalink / raw) To: Stefan Hajnoczi; +Cc: folkert, kvm, virt-tools-list On Fri, Jun 14, 2013 at 12:53:04PM +0200, Stefan Hajnoczi wrote: > On Thu, Jun 13, 2013 at 10:47:32AM +0200, folkert wrote: > > Hi, > > > > > > In virt-manager I saw that there's the option for cache writeback for > > > > storage devices. > > > > I'm wondering: does this also make kvm to ignore write barriers invoked > > > > by the virtual machine? Looking at current git, the cache types supported by virt-manager are: - none - writethrough - writeback - default [virt-manager only, not in virt-install] These translate directly into the libvirt <driver ... cache="..."> field which you can find documented here: http://libvirt.org/formatdomain.html#elementsDisks As far as I can tell (from looking at libvirt sources) as long as you have a modern qemu these will translate to the same names on the qemu command line. > > > No, that would be unsafe. When the guest issues a flush then QEMU will > > > ensure that data reaches the disk with -drive cache=writeback. > > > > Aha so the writeback behaves like the consume harddisks with write-cache > > on them. In answer to the original question by 'folkert': > > In that case maybe an extra note could be added to the virt-manager > > (excellent software by the way!) that if the client vm supports > > barriers, that write-back in that case then is safe. Agree? I suspect the problem with doing this is it depends on the hypervisor. Likely for qemu and Xen (since it uses a qemu device model) this would be true. Possibly not for other hypervisors that virt-manager can control. Generally speaking, it would be nice to document these properly and also how they are implemented in different hypervisors, because I know I for one don't find these settings very obvious. So, patches welcome! Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: cache write back & barriers 2013-06-12 8:03 cache write back & barriers folkert 2013-06-13 8:26 ` Stefan Hajnoczi @ 2013-06-13 14:40 ` Alexandre DERUMIER 1 sibling, 0 replies; 6+ messages in thread From: Alexandre DERUMIER @ 2013-06-13 14:40 UTC (permalink / raw) To: folkert; +Cc: kvm >>I'm wondering: does this also make kvm to ignore write barriers invoked >>by the virtual machine? no, cache=writeback is ok, write barriers are working correctly only with cache=unsafe,it doesn't care about write flush. ----- Mail original ----- De: "folkert" <folkert@vanheusden.com> À: kvm@vger.kernel.org Envoyé: Mercredi 12 Juin 2013 10:03:10 Objet: cache write back & barriers Hi, In virt-manager I saw that there's the option for cache writeback for storage devices. I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? regards, Folkert van Heusden -- Always wondered what the latency of your webserver is? Or how much more latency you get when you go through a proxy server/tor? The numbers tell the tale and with HTTPing you know them! http://www.vanheusden.com/httping/ ----------------------------------------------------------------------- Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html ^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2013-06-16 12:06 UTC | newest] Thread overview: 6+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2013-06-12 8:03 cache write back & barriers folkert 2013-06-13 8:26 ` Stefan Hajnoczi 2013-06-13 8:47 ` folkert 2013-06-14 10:53 ` Stefan Hajnoczi 2013-06-16 12:06 ` [virt-tools-list] " Richard W.M. Jones 2013-06-13 14:40 ` Alexandre DERUMIER
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.